list分组------根据某一个字段分组

Posted 2020-12-24 william_zhao

List分组--对象List分组--MapList分组

List里面的对象元素，以某个属性来分组，例如，以id分组，将id相同的放在一起

//List 以ID分组 Map<Integer,List<Apple>>
Map<Integer, List<Apple>> groupBy = appleList.stream().collect(Collectors.groupingBy(Apple::getId));
 
System.err.println("groupBy:"+groupBy);
{1=[Apple{id=1, name=‘苹果1‘, money=3.25, num=10}, Apple{id=1, name=‘苹果2‘, money=1.35, num=20}], 2=[Apple{id=2, name=‘香蕉‘, money=2.89, num=30}], 3=[Apple{id=3, name=‘荔枝‘, money=9.99, num=40}]}

List里面的Map元素，以某个属性来分组

public static void main(String[] args) {
        List<Map<String,Object>> agentList=init();
        HashMap<String, Object> hashMap =(HashMap<String, Object>) agentList.get(0);
        Map<String, List<Map<String, Object>>> gslist = agentList.stream().collect(Collectors.groupingBy(e -> e.get("sex").toString()));
        for(String key:gslist.keySet()){
            System.out.println(key+" : "+gslist.get(key).size());
            System.out.println(gslist.get(key));
        }
    }
 
    
    
    
    
    /***
     * 初始化联系信??
     * @return
     */
    public static  List<Map<String,Object>> init(){
        String insertID=UUID.randomUUID().toString().replaceAll("-","");
        List<Map<String,Object>> concacts= new ArrayList<Map<String,Object>>();
        long time1=System.currentTimeMillis();
        for(int i=0;i<10;i++){
            String id=UUID.randomUUID().toString().replaceAll("-","");
            Map<String, Object> map = new HashMap<String,Object>();
            map.put("id", id);
            map.put("name", "张三");
            map.put("identity_no", "36242519961225"+(int)(Math.random()*10000+1000));
            map.put("telphone","1852562"+(int)(Math.random()*10000+1000));
            map.put("address","江西吉安");
            map.put("levels", "VIP");
            map.put("source", 0);
            map.put("flight_no", "ZH9101");
            map.put("planned_takeofftime", "1220");
            map.put("estimated_takeofftime", "1425");
            map.put("flight_change_type", 1);
            map.put("flight_change_reason", "军事活动");
            map.put("flightdate","2019-05-01");
            map.put("en_name", "ZHANG/SAN");
            map.put("traveller_idx", (int)(Math.random()*1000+100));
            String [] sexs={"男","女","同性恋","人妖"};
            int kk=(int) (Math.random()*4);
            map.put("sex", sexs[kk]);
            map.put("phone_num","1302880"+(int)(Math.random()*10000+1000));
            map.put("originating", "SZX");
            map.put("terminus", "BKK");
            map.put("ticketid", (int)(Math.random()*10000+1000));
            map.put("mainspace", "J");
            map.put("sonspace", "C");
            map.put("message_info", "4");
            map.put("extension", "1892562"+(int)(Math.random()*10000+1000));
            map.put("officeid", (int)(Math.random()*10000+1000));
            map.put("pnrics", (int)(Math.random()*10000+1000));
            map.put("traveller_safe", "2019-02-23 ZH9007");
            map.put("phone_inform", 1);
            concacts.add(map);
        }
        long time2=System.currentTimeMillis();
        //System.out.println("初始化数据花??"+(time2-time1)/1000);
        return concacts;
    }    

过滤filter

从集合中过滤出来符合条件的元素：

//过滤出符合条件的数据
List<Apple> filterList = appleList.stream().filter(a -> a.getName().equals("香蕉")).collect(Collectors.toList());
 
System.err.println("filterList:"+filterList);
[Apple{id=2, name=‘香蕉‘, money=2.89, num=30}]

求和

将集合中的数据按照某个属性求和:

//计算 总金额
BigDecimal totalMoney = appleList.stream().map(Apple::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);
System.err.println("totalMoney:"+totalMoney);  //totalMoney:17.48